Basically the error is related to the UI. The message appearing in the output may not reflect the code that actually triggered it – so to debug you need to isolate the code that’s causing it. Your first port of call should be callbacks from alerts, or other popups. For me it was the UIImagePicker, I fixed it like so:

The pertinent code is the BeginInvokeOnMainThread block, this is fixing the wait_fences issue. It looks like the callback method isn’t being executed on the UI thread. I’m not sure if this is an iOS SDK thing, or a quirk added by MonoTouch. It could also be that the picker is still animating away when I start tweaking the UI, causing the issue. For example messing about with the UI in the Clicked event of a UIAlertView can sometimes cause minor issues, moving the code to the Dismissed event fixes the issues completely.

It’s quite common to have a ‘More’ button at the end of long lists within a LongListSelector. The problem is the template inexplicably doesn’t cascade the DataContext, so you have no access to the collection to trigger a more command. Fortunately the workaround is simple, albeit you can’t use pre-defined DataTemplates.

Recently I had a problem with Isolated Storage Settings on WP7 – the entire collection appeared to be wiped every single time I restarted the app. The problem turned out to be the deserialization stage of IsolatedStorageSettings, in essence if any object can’t be re-instantiated, then the entire collection is inaccessible. The collection will work correctly until you attempt to retrieve something that can’t be instantiated.

At the bare minimum the constructor of your class must be public.

All non-public properties will also fail to be restored, resulting in a rather empty object instance when you come to load the values that should have been saved (and have been… they just don’t come back). Be very careful with this issue, you may not notice the problem until it’s too late.